10 plaxis bulletin (s)

PLAXIS
PLAXIS
PLAXIS
Editorial
N 10
- MARCH 2001
Since the release of the Dynamics module last
year users have been running dynamic
In April the newly developed 3D Tunnel
analyses. As this is a new module of Plaxis,
program will officially be released . Many
many users have to gain experience with it.
months were spent in testing the program,
Therefore, the Plaxis Users Association (NL)
not only at the Delftech park office, but
organised a well attended Dynamics day with
also by a group of Beta testers. The 3D
guest speakers Prof. dr. ir. A. Verruijt and T.K.
Tunnel program is specifically intended to
Muller from IFCO. After these interesting
model tunnels such as shield tunnels
lectures, hands-on exercises were conducted
(second Heinenoord Tunnel, see New
in the afternoon to exchange practical
Developments) and NATM tunnels. But the
experience. Last January, during the course
3D Tunnel program allows other 3D
Computational Geotechnics at Berkeley
situations to be modelled as well, such as
University (USA) a one day Dynamics course
the 3D excavation and installation of a
was introduced. Some 30 participants from this
diaphragm wall (M. de Kant, see Plaxis
earthquake sensitive area were present.
Practice). A short description of the 3D
Tunnel program is provided in this bulletin.
The analysis of flexible soil retaining walls is
taken a step further. From Blums analysis and
L
Winkler spring type models to an analysis in
3D Tunnel program was presented at a users
Plaxis. The approach of using a Finite Element
method with an adequate soil model to analyse
background of the 3D Tunnel program, a
flexible soil retaining walls shows that some
hands-on exercise was given on a simple
interesting results can be obtained (Column
example problem. Further more it was shown
Plaxis bulletin
Plaxis B.V.
P.O. Box 572
2600 AN Delft
The Netherlands
E-mail:
bulletin@plaxis.nl
ast year, in September, a Beta release of the
meeting. Besides a lecture on the (theoretical)
Bulletin of the
PLAXIS
Users Association (NL)
Vermeer).
that the PLAXIS 3D Tunnel program is certainly
capable of other analyses beyond tunnelling.
A case about the indentation of a tractor wheel
about the role of OCR in the model. A practical
excavation of a slurry wall were presented.
Editorial
implemented, questions have been asked
in soft soil (University of Wageningen) and the
IN THIS ISSUE:
Since the Soft Soil Creep model has been
example is given and preliminary conclusions
Column Vermeer
are drawn in The role of OCR in the SSC model
see Plaxis Practice.
New developments
Improved user
services support
Editorial staff:
Recent activities
Marco Hutteman, Plaxis Users Association (NL)
Martin de Kant, Plaxis Users Association (NL)
Peter Brand, Plaxis bv
Plaxis Practice
Jan Gabe van der Weide, Plaxis bv
The role of OCR
12
Users forum
14
Agenda
16
Scientific Committee:
Prof. Pieter Vermeer, Stuttgart University
Partial geometry of NATM tunnel
1
Dr. Ronald Brinkgreve, Plaxis bv
PLAXIS
PLAXIS
Column Vermeer
FE-computations. We will consider a singleanchored wall for three different cases.
ON SINGLE ANCHORED RETAINING WALLS
We considered the geometry of Fig. 1, i.e. an
The analysis of flexible soil retaining walls
excavation depth of 10 m, an embedment of
became possible through the work of Blum
2.5 m and an anchor at a depth of 2.5 m. The
in the 1930s. Considering single-anchored
anchor force was given a fixed value of
or single-propped sheet-pile walls, he
100 kN/m.
distinguished between two types of
For all three different cases (A, B and C) the
embedments:
following soil properties were adopted:
q
free earth support
q
fixed earth support
Submerged soil weight was used, as we
ree earth support implies a relatively short
F
consider a water table at the soil surface, being
wall with minimum embedment. Fixed
not lowered at all, i.e. neither in front nor
earth support implies a somewhat larger
behind the wall. The excavation was done in
embedment. According to Blums definition,
three stages of construction:
full fixity is achieved when the fixity moment
1 Installation of wall and excavation to a depth
equals the field moment.
of 2.5 m
2 Application of anchor load of 100 kN/m
Blums design procedures for retaining walls
3 Excavation down to final depth
with free or fixed earth support can be found
in most textbooks. In the authors opinion they
Hardening soil model: Soil behaviour was
constitute outstanding contributions to Soil
simulated using the HS-Model of the Plaxis
Mechanics. However, as Blums analysis involves
code. For virgin oedometer loading, this
neither the wall stiffness nor the soil stiffness
implies an increasing tangent stiffness modulus
it is bound to be inaccurate. As a consequence,
according to
one is now mostly using Winkler spring type
with
models. Unfortunately it is difficult to select
appropriate spring constants and I would rather
use the FE method. To assess the impact of
where
stiffnesses we decided to perform a series of
adopted the exponent m = 0.5. Within the HS-
is the major principal stress. We
Model unloading-reloading is described on the
Figure 1
Single-anchored wall
with free earth support
with 3 stages of
construction: first
excavation, anchoring
and final excavation. In
practice anchors will be
installed just above the
groundwater table.
basis of Hookes law. Youngs unloadingreloading modulus for increments of stress
and strain reads:
Table 1 Stiffness parameters.
where
is the minor principal stress. For all
analyses, the over-consolidation ratio was taken
to be OCR = 1.0 and initial stresses were
computed using Ko = 0.5. The HS-Model also
allows for a specification of soil stiffness in
drained standard triaxial tests. For all analyses,
we used
2
PLAXIS
PLAXIS
The only difference between the stiff soil of
The simulation of arching behind a flexible wall
Cases A and B, and the soft soil of Case C relates
makes the FEM superior to subgrade reaction
to the stiffnesses. The stiff soil is simply a factor
type models. In the latter case the spring will
15 stiffer than the soft soil, but the relation
yield plastically as soon as
eah is reached and
is 1/1/4 for both soils. Moreover,
active pressures will never reach smaller values
both the stiff and the soft soil are conveniently
given the same strength parameters.
than eah= kah . . Fig. 2 clearly demonstrates
z
the significance of arching, as computed active
Case A: Considering the FE-results for the
earth pressures are well below the dashed line
combination of a stiff soil and a flexible wall,
for eah. It happens for flexible walls in stiff soils.
one observes in Fig. 2a considerable wall
bending up to about 5 cm. As a consequence,
the active earth pressures reduce significantly;
stiffness is the fixity of its base. There is a
even below the classical minimum of e ah .
Indeed, plots of stresses showed significant
Figure 2
Single-anchored wall
with free earth support.
Another feature of a wall with low relative
significant fixity moment! Here it should be
arching between the anchor and the passive
length using Blums design rules for a wall with
pressure below the bottom of the excavation.
no fixity at all. Due to the significant amount
noted that we computed the embedment
of arching and the base fixity, computed
bending moments are small; approximately
half the ones that would follow from Blums
design rule.
Case B: Typical Blum-type results are obtained
when considering a stiff wall in a stiff soil (Case
B). Below the anchor classical active earth
pressures are reached. The passive ones are
not fully mobilised, as we designed the wall for
a factor of safety of 1.5 on the passive earth
pressure. The base of the wall shows no fixity
at all and bending moments agree well to the
ones that follow from Blums analytical design
procedure. Please note that the same earth
pressures and bending moments would have
been obtained for the combination of a soft
soil and a flexible wall. In such a case we would
have the same relative wall stiffness as for the
stiff-stiff combination of Case B.
Case C: I was amazed when considering
computational results for a stiff wall in a soft
soil. Despite the use of a factor of safety of 1.5,
the passive earth pressure is nearly completely
mobilised. It appears to be caused by an
enlarged active pressure. Apparently, the soil
is so deformable that wall displacements of
about 5 cm are insufficient for a proper
reduction of pressure on the active side. As a
consequence of the high pressure a bending
moment of nearly 300 kNm/m occurs. No
doubt, this is well beyond the values that would
3
PLAXIS
PLAXIS
follow from Blums design analysis.
5 m is considered. Following Blum s design
rules this would yield full base fixity, such that
For a stiff wall in soft soil, I would also doubt
the fixity moment equals the field moment.
the results of subgrade reaction type
Computational results for all three different
calculations, as this method suffers from the
relative wall stiffnesses are shown in Fig. 3. For
difficulty of selecting proper spring constants.
comparison, previous data for the shorter wall
Realistic values would be required both for the
are indicated by dashed lines.
active and the passive zone; otherwise it is
It appears from Fig. 3 that bending moments
impossible to predict the high bending
are only slightly reduced when increasing wall
moments as obtained for a stiff wall in soft
length. This is surprising as some textbooks
soil.
suggest a significant effect on the bending
moments. Considering present computational
Embedment: For studying the effect of
Figure 3 Singleanchored wall for fixed
earth support. Dashed
lines indicate results for
free earth support.
data, we conclude that bending moments are
embedment, we reconsider the wall of Fig. 1,
in general not significantly reduced by
but now the penetration depth is doubled.
increasing wall penetrations. Present data
Hence, instead of 2.5 m, an embedment of
show, that the reduction of the field moment,
as caused by the fixity moment, is more or less
compensated by a slight increase of active
pressure, as caused by the stiffening of the
entire system. Deep penetration is neither of
great import when considering displacements.
Indeed,
a
significant
reduction
of
displacements is only achieved for Case A.
Conclusions: When considering a stiff wall in
a stiff soil (Case B) typical Blum-type results are
obtained. In this case classical active earth
pressures will occur, at least below the anchor.
Obviously, the passive ones will not be fully
mobilised, if the wall is designed for a factor
of safety equal of 1.5 on the passive earth
pressure, as done in the present example.
A flexible wall in a stiff soil (Case A) will result in
considerable wall bending and low bending
moments. The stiff soil transfers a large part
of the active pressure by arching and the
flexible wall gets a relative small pressure.
A stiff wall in a soft soil (Case C) will result in
high active pressures and, as a consequence,
high bending moments.
Finally we conclude that bending moments are
in general not significantly reduced by
increasing wall penetrations.
P.A. Vermeer, Stuttgart University
4
PLAXIS
PLAXIS
New Developments
monitored.
Calculations
of
different
construction phases are performed for the
The Plaxis 3D Tunnel program is about to
North bank. In a 3D finite element model (one
be released. In the previous Bulletin it was
symmetric half) the sub-soil, the Tunnel Boring
explained why this first 3D Plaxis program
Machine (TBM) and a part of the final lining were
is devoted to tunnels. At the moment,
modelled according to the Grout pressure
quite some engineering and research is
modelling procedure (see Fig. 1). The sub-soil
focused on tunnelling, both NATM and
was schematised by means of 8 layers, with
shield or bored tunnelling. Tunnelling
their location and properties as listed in Table
involves three-dimensional aspects that
1. All layers were modelled using the Mohr-
cannot be analysed with conventional
Coulomb model. The layers located under the
methods. Hence, there is a demand for a
tunnel were given a high unloading stiffness.
3D design model for tunnels. Nevertheless,
creative users of the Plaxis 3D Tunnel
The hydrostatic pore pressure distribution for
program may find many other applications
all layers was determined from a phreatic level
in addition to the analysis of tunnels.
at +1.0 m.
n the past few months, beta-testers have
I
The 3D finite element model consists of 3440
used a pre-release of the 3D Tunnel program
quadratic volume elements divided over a
in practical applications. Some of these
number of slices (see Fig. 2). Each slice is 3.0
preliminary results are presented in this
m in the longitudinal tunnel direction. The TBM
was modelled over 3 slices and composed of
shell (plate) elements, with a flexural rigidity
Table 1 Soil layers and parameters used in the Mohr-Coulomb model
Layer Top
Type
m
unsat
kN/m3
E
sat
kN/m3 -
c
kN/m2
K0
kN/m2
EI = 50103
kNm2/m, a normal stiffness
EA = 10106 kN/m and a weight w = 38,15
kN/m2. The radius of the TBM is 4.25 m and its
MSL
17.2
27.0
0.0 0.58
0.35 m thick concrete tunnel lining was
6.5 0.47
modelled using volume elements with the
3.0 0.47
following properties:
5.0 0.45
24.6106
2
1.00
Drained
16.5
17.2
0.34
3900
3.0
3
-1.50 Drained
20.5
20.5
0.30
29600
0.0
4
-5.75 Drained
19.0
19.0
0.31
18500
0.0
33.0
0.0
27.0
35.0
kN/m2,
= 24 kN/m3, E =
n = 0.2.
5
-10.00 Drained
19.5
6
-17.25 Drained
20.5
20.5
0.30
444000 0.0
36.5
6.5 0.50
7
-20.75 Undrained 20.0
20.0
0.32
119000 7.0
31.0
1.0 0.55
The tunnel boring process was modelled
8
-25.00 Drained
21.0
0.30
593000 0.0
37.5
7.5 0.56
according to the Grout pressure modelling
21.0
19300
3.0
36.5
Undrained 16.5
0.30
3900
centre point is located at -12.3 m MSL. The
2.50
19.5
0.34
0.0 0.58
1
procedure as schematised in Fig. 1.
Bulletin. In this article I will shortly present some
results of a 3D calculation for the Second
Heinenoord Tunnel, the first large-scale bored
tunnel project under soft soil conditions in the
South-West of The Netherlands.
The situation at the Second Heinenoord Tunnel
is described in various publications (see
Figure 1 Modelling aspects in Grout
pressure modelling procedure.
References). The tunnel is formed by two tubes
with outer diameters of 8.5 m, which were
A front pressure was applied at the bore font
bored under the river Oude Maas. In order to
to support the soil. The front pressure is 140
gain experience with tunnel boring under soft
kN/m2 at the top of the TBM and 259 kN/m2
soil conditions, the situation was extensively
at the bottom. The TBM is conical. The tail
5
PLAXIS
PLAXIS
radius is 2 mm smaller than the front radius,
m (TBM 3 slices, liquified grout zone 2 slices,
which corresponds with a contraction of about
tunnel lining 9 slices). The results of the
0.48% (0.16% per slice per phase). Behind the
calculation at the end of Phase 14 are
TBM grout is injected in the tail void. It is
presented underneath.
assumed that the grout remains liquified over
2 slices (6 m), which results in a grout pressure
Fig. 2 shows the deformed mesh. This plot
on the surrounding soil. The grout pressure is
clearly shows the settlement trough at the
kN/m2
at the
ground surface, with a maximum settlement
bottom. Behind the liquified grout zone the
of about 22 mm. The results are quite realistic
tunnel lining is activated and jack forces are
and correspond reasonably well to the
applied in backward direction (varying from
measurements. This statement also applies to
125
Figure 2 Deformed
mesh at the end of
Phase 14 (deformations
50 times enlarged).
3365
at the top and 190
kN/m2
kN/m2
at the top and
6731kN/m2
at the
the width of the settlement trough.
bottom).
Calculations using contraction only tend to
In the initial situation, initial stresses are
overestimate the width of the settlement
trough, whereas calculations according to the
grout pressure modelling procedure give
more realistic results. The deformations just
above the tunnel lining are somewhat larger
than at the settlement surface (max. 38 mm).
Fig. 3 shows the shadings of total displacements. This plot confirms the above and clearly
shows where the larger displacements occur
(just above the lining). In this plot it can also be
seen that the buoyancy of the tunnel is
relatively little, since the displacements below
the tunnel are small.
In addition to the displacements, the stresses
can be visualised in the full 3D mesh as well as
in individual and user-defined cross sections.
From such plots the three dimensional arching
around the tunnel can be viewed. There are
also several possibilities to show the forces and
deformations of the TBM and the tunnel lining,
both in 3D and per cross section. The
maximum bending moment in the TBM is
around 100 kNm/m and the maximum
bending moment in the lining is around 80
kNm/m. These values are also quite realistic.
From the results it can be concluded that it is
Figure 3 Shadings of
total displacement at the
end of Phase 14
(displacements 50 times
enlarged).
very well possible to calculate the three
generated by means of the K0-procedure,
dimensional effects around bore tunnels and
using K0-values as listed in Table 1. The whole
to accurately predict surface settlements using
calculation is divided into 14 phases. In Phase
the grout pressure modelling procedure.
1 the TBM enters the model in the first slice
The above analysis took some 6 hours to
and the process advances 1 slice in each phase.
calculate on a Pentium III 500 Mhz with 768 MB
In the final Phase 14 the TBM has advanced 42
RAM.
6
PLAXIS
PLAXIS
Improved User
References:
[1] Bakker K.J., van Schelt W., Plekkenpol J.W.
(1996), Predictions and a monitoring
Support Service
scheme with respect to the boring of the
Second
Heinenoord
In:
As you may have read in the last Plaxis
Geotechnical aspects of underground
bulletin, the Plaxis company is growing.
construction in soft ground, (eds: R.J. Mair
Currently the staff consists of 10 people.
and R.N. Taylor). Balkema, Rotterdam. pp.
This means we can further increase the
459-464.
quality and the range of products. In April
[2] Jaarsveld
E.P.,
Tunnel.
Plekkenpol
J.W.,
this will first show by the release of the 3D
Messemaeckers van de Graaf C.A. (1999),
Tunnel program. Obviously these new
Ground deformations due to the boring
products and the increasing group of Plaxis
of the Second Heinenoord Tunnel. In:
users (now over 2000 professionals !) require
Geotechnical
for
good user services. Last year we employed
transportation infrastructure (eds: F.B.J.
a full time support engineer in order to
Barends, J. Lindenberg, H.J. Luger, L. de
keep up with the foreseen demand on
Quelerij, A. Verruijt). Balkema, Rotterdam.
profession support.
infrastructure
pp. 153-159.
Tweede Heineneoordtunnel, verslag van
W
een groootschalig praktijkonderzoek naar
Obviously such projects have a deadline. Hence,
geboorde tunnels. Final report COB
it is important that users can rely on a good
committee K100. CUR / COB, Gouda.
adequate HelpDesk. Not only on an operational
[3] CUR / COB (1999), Monitoring bij de
e realise that most support questions
occur while working on a project.
Validatie
level, but also on a more scientific level. To ensure
Groutdrukmodel, Meetveld Noord. Report
urgent questions get the highest priority, we are
of COB committee L520. CUR / COB, Gouda.
introducing two different levels of support.
[4] CUR
/
COB
(1999),
3D
[5] Bakker K.J. (2000), Soil Retaining Structures;
development of models for structural
Support service level 1:
analysis. Dissertation (Delft University of
This service level is free of charge and provides
Technology). Balkema, Rotterdam.
assistance by e-mail within reasonable response
[6] Peters R., Safari B. (2000), 2D Modellering
times, obviously depending on the support
van het tunnelboor- en consolidatieproces.
demand. No telephone and scientific assistance
Internal
is provided within this service level.
report
BSRAP-R-00004,
Bouwdienst Rijkswaterstaat, Utrecht.
[7] CUR / COB (2000), Toetsingsrichtlijn voor
Contact HelpDesk:
e-mail:
support@plaxis.nl
het ontwerp van boortunnels voor wegen railinfrastructuur. Final report COB
Support service level 2:
committee L500. CUR / COB, Gouda.
The second level of support is to users who
have a PLAXIS software support agreement.
Ronald Brinkgreve,
This agreement provides a professional support
DELFT UNIVERSITY OF TECHNOLOGY
service on operational as well as on a scientific
& PLAXIS BV
level, within 24 hours of normal working days.
Contact HelpDesk:
e-mail:
support@plaxis.nl
fax:
+31 15 2600 451
telephone:
+31 15 2600 450
For more information on the support
agreement, please contact Plaxis BV.
7
PLAXIS
PLAXIS
Recent Activities
courses. It is also good to realise that the local
courses are organised and lectured in
Plaxis 3D Tunnel program
cooperation with different local experts, local
As this bulletin shows, the Plaxis 3D Tunnel
university professors as well as professionals
program is nearly finished. Currently the Plaxis
from the engineering practice. This formula
team puts the finishing touch to the 3D Tunnel
creates tailor made courses, providing
program and the final release is expected in
theoretical and practical backgrounds on the
April this year. The 3D Tunnel program is
use of Plaxis. The courses in the Netherlands
specifically intended to model shield, bored
reflect the Plaxis philosophy with respect to
and NATM type of tunnels. Hence the program
lecturing and the usage of the Plaxis program.
has special features such as a state-of-the-art
The contents of the courses therefor are used
tunnel designer, realistic grout pressure
as the blue-prints of all international course.
modelling, staged construction, Jointed rock
model, etc. The userfriendlyness of the input,
During the past course in Noordwijk, the
the
and
Netherlands, we welcomed 41 participants
robustness of the numerical procedures are in
from virtually all over the world. Seventeen
line with the Plaxis 2D software. This means
different nationalities, including people from
creating the finite element model and
Japan, Jamaica, Mexico, Brazil, Israel, South
prescribing the calculation phases are relatively
Africa, just to mention a few. This is a clear
simple, especially when compared to the
indication of the true International character
efforts it takes using general purpose FE codes.
Plaxis is enjoying.
automatic
mesh
generation
Such codes are usually command driven and
lack advanced options needed for the
modelling of soil, structures and soil/structure
interaction. With this new tool, Geotechnical
3D
calculations
can
become
generally
applicable.
Figure 2 Participants and lecturers in the
Noordwijk course.
So far some 10 new courses have been
scheduled for this year. At the upcoming
courses in Noordwijk, new lectures dedicated
to both Dynamics and 3D modelling are
presented (see also Agenda).
Plaxis Practice
Figure 1 The Plaxis 3D
Tunnel Program
RESEARCH PROGRAM ON THE IMPACT OF
DIAPHRAGM WALL INSTALLATION.
Courses
VALIDATION OF A 3D-FE MODEL.
In the past year 11 courses on Computational
Geotechnics were lectured at different
1. Introduction
locations all around the world. Six in Europe,
The North-South metro line in Amsterdam will
three in Asia, one in the US and one in the
connect the northern and southern suburbs
Middle East. From the agenda in this bulletin
with the city centre (De Wit, 1998). For reasons
you can see we plan to continue lecturing such
of protecting the historic city centre and
8
PLAXIS
PLAXIS
restricting the disruption of city life, a bored
tunnel will be applied that follows the street
q
measurements of stress in the trench
(pizometers on reinforcement cages).
pattern as closely as possible and is lowered to
a great depth. Consequently the underground
The geotechnical profile is comparable with
stations are at a great depth as well. The
the locations of the future stations of the
stations will be constructed in a building pit
North-South line, see table 1.
with 40m long braced diaphragm walls. One of
the most important aspects in the design of
Table 1. Geotechnical profile en
the stations is the impact of construction on
soil-parameters.
historical buildings. Because knowledge on the
Type of soil
Top level
NAP (m)
impact of diaphragm wall construction on
CPT
(Mpa)
surrounding buildings is only limited, it was
Fill (sand)
+2,0
10-15
decided to carry out a research project,
Clay
-1,0
0,5
consisting the following four phases:
Peat
-3,5
0,5
1 prediction of the impact with a 3D FE-
Clay, silt
-7,0
0,5-1,0
model;
Peat
-13,0
1,5
2 full scale test;
Sand
-14,0 1)
8-30
3 interpretation of test results;
Silty sand
-17,0
1-5
4 validation of the 3D FE-model.
Clay (eemklei)
-25,0
1-5
This paper focuses on phase four.
Sand
-28,0
10-30
Silt
-42,0
3
2. Full scale test
1) =
The test was carried out at the construction site
Foundation layer of ancient
buildings on wooden piles
of the Mondriaan Tower, where diaphragm wall
panels are applied as foundation elements as
3 FE - validation calculations
well as building pit walls. Figure 1 gives an
The main goal of the validation calculations is
impression of the test site. During the different
developing a model which can be used to
stages of construction of 5 panels, a monitoring
predict the soil displacements during the
program was conducted consisting of:
construction of a diaphragm wall. The
Fig. 1 Impression of
the test-site
calibration was carried out, mainly by changing
the soil parameters within a certain bandwidth.
(extensometers, inclinometers);
q
measurements of vertical and horizontal
displacements in the surrounding soil
q
This calibration procedure has resulted in a
settlement and bearing capacity tests on
best-fit
piles;
calculations were made with the official
calculation
(BF).
Additionally,
North/South-line parameter-set (NS), which
has also been used in the 2D tunnel- and
building-pit calculations. In both calculations,
BF and NS, the hard-soil model is used.
Element mesh and modelling procedure
For the 3D-calculations a preliminary 3D-version
of PLAXIS (6.4b) has been used. The mesh is
build of 3D-wedge-elements with 15 nodes
and 6 Gauss-points. For the calculation of one
panel a mesh with approx 5,000 nodes has
been used (fig. 2). Only a quarter of the panel
is modelled, which means that there are two
planes of symmetry: in plan view x- and zdirection.
9
PLAXIS
PLAXIS
Fig. 2 Finite
element mesh
The construction of a single diaphragm wall
panel is modelled using the following stages:
1 Excavate the single trench by switching the
soil elements off and, simultaneously,
=d.
applying the bentonite pressure (
b
is hardened.
Above the hardened concrete, the wet
= d . ) acts on the
concrete pressure (
c
faces of the trench (
c
c
= volume weight of wet
concrete). The lateral pressure in step 2 can
therefore be described by a bilinear relation:
were hcrit is a critical depth which depends on
the concrete placing rate, cement type,
temperature etc. (Lings et al 1994). In case of
a constant rate of concrete placing,
to
is equal
b.
During installation the stresses in the soil will
initially decrease (stage 1) and, subsequently,
increase due to the wet concrete pressure
(stage 2). The stresses decrease exponential
with increasing distance from the trench.
Among these stress-paths the stiffness of the
soil varies strongly. The calculations where
therefore carried out using non-linear material
models: the hard soil model (HSM) and the
soft-soil model (SSM). Only the elements
representing the clay and peat layers are
modelled as undrained.
Calculation results for a single panel
Figure 3 shows the horizontal effective
stresses, at a depth of NAP-15m, during the
different stages (1 and 2). The initial horizontal
stress is about 50 kPa. During excavation the
stress at the centre of the panel decreases to
about 25 kPa. Due to horizontal load transfer
(arching), at the edges of the panel, the stress
increases and exceeds K0 situation. After
concreting, the stress at the centre increases
significantly. At the corner a decrease of
effective stress occurs.
The calculations have shown that load transfer
not only acts horizontal but also in vertical
Fig. 3 Horizontal
effective stresses in
different stages
b)
on the faces of the trench (d = depth,
b = volume weight of bentonite).
2 Fill the trench with concrete by increasing
direction
to
relative
stiff
layers.
The
displacements in layers beneath those stiff
layers therefore decrease and are no longer of
the lateral pressure.
influence on displacements of the top layers.
Directly after pouring the trench, from
Therefore the influence of the panel-depth in
bottom up, the lower side of the concrete
a layered subsoil with stiff layers is very small
10
PLAXIS
PLAXIS
which was an important conclusion of the
prediction calculations.
The model was calibrated on the displacements
in the concrete-phase (stage 2), because
displacements during the excavation phase
(stage 1) were to small.
Figure 4 shows the horizontal displacements
at the centre of the panel, at approx. 2m from
the trench. The maximum displacement occurs
in the Holocene top-layers, particularly the peat
layer at NAP-6m. The BF-calculation is in good
agreement with the test-results. Regarding the
top layers, the NS-calculation is also accurate,
Fig. 4 Horizontal displacements,
middle of the panel
but the horizontal displacements at lower
layers are over predicted. The computed
vertical maximum displacements of the sandlayer at a level of NAP-15m (foundation layer
of wooden piles) are for both the BFcalculation and the NS-calculation in good
agreement with the test-results (fig 5). At the
test-site a settlement bowl occurs with a
maximum settlement at a distance of 3m from
the trench. This is in good agreement with the
NZ-calculation, although the calculations shows
a much wider settlement bowl.
Influence of panel size and installation
sequence
At the test-site 5 panels were monitored, with
different sizes and shapes. In contrast to the
Fig. 5 Vertical displacements on
level NAP -15m
calculations and literature, the panel width was
of little influence on the displacements. The
reason for this discrepancy has probably been
the installation sequence of the panels. The
small panel (b=2,7m) was excavated in a
undisturbed situation whereas the wide panel
(b=6,4m) was excavated between two former
installed panels. Those adjacent panels causes
a reduction of displacements because of:
q
hardening (due to the installation of the
adjacent panels the soil is overconsolidated);
q
load transfer to adjacent stiff concrete walls.
To confirm this interpretation, the described
situation was computed with the calibrated
model (NZ-parameters). Figure 6 shows the
Fig. 6 Influence of panel size
and sequence of installation on
horizontal displacements
horizontal displacements of undisturbed panels
with different widths, and a wide panel
11
PLAXIS
PLAXIS
between to installed small panels. Although
hardening was not calculated due to the
undrained behaviour, the results are in
The role of OCR in
the SSC Model
agreement with the expectation.
The Over-Consolidation Ratio (OCR) is
Conclusions
defined
The diaphragm wall research project has
preconsolidation stress
enlarged the understanding of the impact of
in-situ stress: OCR =
D-wall installation on surrounding soil. Main
parameter that indicates the amount of
conclusions are:
overconsolidation of the soil. As long as the
as
the
ratio
p
between
the
p and the effective
/ yy. OCR is a state
soil behaviour is stiff. This applies to
the ground deformations were relatively
unloading as well as to reloading. As soon
as the existing preconsolidation stress is
displacements in the soft top layers;
passed by primary loading of the soil, the
the width of a diaphragm wall panel is of
soil
great influence on ground displacements.
preconsolidation stress further increases
Due to vertical load transfer, the influence
along with the effective stress level. In that
of the panel-depth in a layered subsoil with
case the soil is said to be in a state of
stiff layers, is relatively small;
normal consolidation, which would imply
the computed displacements with the
OCR=1.0. This is exactly what happens in
calibrated 3D-FE model are in good
Cam-Clay type of models, like the Soft Soil
agreement with the test -results.
q
preconsolidation stress, i.e. OCR>1.0, the
small, with exception of the horizontal
q
effective
nearby wooden pile foundations;
q
the installation of a diaphragm wall in a
layered subsoil has no significant impact on
q
model in Plaxis.
stress
behaviour
is
is
soft,
below
whilst
the
the
Ingenieursbureau Amsterdam
A
North/South Line Design Office,
Plaxis. In that respect the meaning of the OCR-
Amsterdam, The Netherlands
value is similar, although the transition from
similar behaviour can be observed when
ing. M. de Kant
using the Soft Soil Creep (SSC) model in
the stiff reloading behaviour to soft primary
Literature
loading is more gradual (see Fig. 7.13 of the
Lings, M.L. & C.W.W. Ng. & D.F.T. Nash, 1994,
Plaxis Material Models Manual [1]). In order for
The lateral pressure of wet concrete in
the preconsolidation stress to follow the
diaphragm wall panels cast under bentonite,
effective stress level, time is needed. Hence,
Proc. Instn. Civ. Engrs. Geotech. Engng, 163-
when loading the soil very quickly, the OCR-
172.
value can (temporarily) become less than 1.0.
De Wit, J.C.W.M., 1998, Design of underground
On the other hand, if the load remains
stations on the North/South line, Proceedings
constant, the creep process continues in time,
of the World Tunnel Congress, Sao Paolo.
which results in a gradual increase of the
De Wit, J.C.W.M., J.C.S. Roelands & M. de Kant,
preconsolidation stress and OCR (without
Full scale test on environmental impact of
physical loading). The latter process can be
diaphragm
in
conceived as ageing. The idea that ageing
of
would
Amsterdam,
wall
trench
Geotechnical
excavation
Aspects
lead
to
an
increase
of
the
Underground Construction in Soft Ground, IS
preconsolidation stress was introduced by
1999 Tokyo Japan, Balkema Publ.
Bjerrum in his creep formulation [2]. This idea
is also adopted in the SSC model. In the SSC
model the increase of the creep strain in time
12
PLAXIS
PLAXIS
(= creep strain rate) depends (in addition to the
west of The Netherlands) the initial OCR-value
*) on the ratio of the
is, per definition, equal to 1.0. On the other
effective stress and the preconsolidation stress,
hand, even very soft soils often show a
i.e. on the inverse of OCR. Hence, starting from
preconsolidation stress that is over 20 kPa
an initial effective stress state, the initial creep
beyond the in-situ stress level. Particularly in
strain rate depends on the initial OCR-value.
the top layer (just below the ground surface)
modified creep index,
crust forming may occur, which is associated
The latter is not very known among Plaxis
with relatively stiff and strong behaviour. The
users. Since practical situations always involve
reason for this can be drying of the soil,
effective stresses from the very beginning, the
variations of the phreatic level, temporary
creep process (settlement) starts immediately
loads, temperature changes, etc. Nevertheless,
without additional loading, whereas the
these soil are still considered to be normally
settlement velocity depends on the OCR-
consolidated, but the actual OCR-value is often
value. For reasonable combinations of SSC
higher than 1.0.
parameters, the use of a default initial OCRvalue of 1.0 in the K0-procedure may lead to
excessive initial settlement velocities. Hence,
layer under standard boundary conditions with
the initial OCR-value needs to be selected with
properties as listed in Table 1. The parameters
care. An initial OCR-value larger than 1.0 is
Figure 1:
Time-settlement curves
for different initial
values of OCR
Let us consider, for example, a 10 m thick clay
are arbitrary, but realistic for a normally
generally recommended.
consolidated clay. Initial stresses are generated
It is generally said that for normally
with the K0-procedure, using different initial
consolidated soils (like the soft clays in the
values of OCR (OCR0). On using different OCR0values, Plaxis proposes different K0-values, but
all K0-values are reset to their original value of
K0nc=0.703. For all values of OCR 0 a drained
calculation is performed for a total time of
1000 days, just to let the soil creep under its
self weight without additional loading. The
results of these calculations are presented in
Fig. 1.
From Fig. 1 it can be seen that there is a
remarkable difference in settlement, which is
particularly caused by the initial inclination of
the time-settlement curve, i.e. the initial creep
rate. For OCR0=1.0 the initial creep rate is quite
unrealistic. Considering rather soft normally
consolidated soils, it is quite realistic to have
a settlement of 0.05 m a year, decreasing down
to 0.01 m a year when the soil hasnt been
disturbed for a some years. In the above
situation this is well reflected by the choice of
Table 1. Properties of arbitrary clay layer,
OCR0=1.4. At t=1000 days the inclination of all
modelled with the Soft Soil Creep model
wet
17
*
*
*
0.02
0.10
0.005
c
0.15
1.0
K0nc
26.0
0.0
curves is almost the same, except for
0.70
OCR0=2.0, which would suggest that the actual
OCRs have increased to almost the same value.
kN/m3
Evaluation of the actual OCR-values at t=1000
13
PLAXIS
PLAXIS
days gives OCR 1.8 for all cases, except for
Users Forum
OCR0=2.0. In the latter case OCR has hardly
increased.
Question:
I have some difficulties in generating the initial
From the above results it could be concluded
stresses in my project. My project is a tunnel
that an initial OCR-value of 1.4, to be used in
200.0 m underground and is not including the
the K0-procedure, would be a good choice. This
ground surface; otherwise the model will be
could indeed be said for the above example
too big. Therefore, the initial stresses at the
and perhaps for other cases, but it cannot be
top of the model are not zero and should
stated in general. It is a good habit for a
include the overburden above the top of the
practical application to simulate a certain creep
model. Could you please give me a hint solving
period in a drained calculation without
this problem?
additional loading and to evaluate the initial
OCR-value. Please note that when using initial
Answer:
OCR-values larger than 1.0, Plaxis will propose
If the model becomes too large you can
K0>K0
nc,
whereas for normally consolidated
eventually omit the upper part, but you have
soils it is recommended to reset K0=K0nc. If the
to compensate for the missing soil weight
resulting settlement velocity is too high and
otherwise non-realistic stresses will be
the other parameters of the SSC model have
generated. To include an overburden pressure
been properly determined, then the initial value
you have to do the following: Create a thin
of OCR should be increased (or the modified
layer with thickness h at the top of your model
creep index
* should be reevaluated).
representing the omitted soil (see Figure 1).
The weight of the soil in the thin layer is higher
In conclusion, the initial OCR-value in the Soft
than the omitted soil and equal to (
Soil Creep model needs to be selected with
(
real * hreal / hvirtual. For example in Figure 1,
care. In principle, the initial value depends on
(
virtual
the ratio between the initial preconsolidation
this way you can generate realistic initial
stress and the effective in-situ stress (which is
stresses by means of the K0 procedure.
virtual
=
= 18.0 * 130.0 / 1.0 = 2340.0 kN/m3. In
generally slightly larger than 1.0), but it should
be realised that the initial OCR-value also
determines the initial settlement velocity. A
first estimate could be OCR=1.4, but it is
advisable to simulate the creep process and
adapt OCR if necessary.
RONALD BRINKGREVE,
DELFT UNIVERSITY OF TECHNOLOGY
& PLAXIS BV
References
[1] Brinkgreve R.B.J., Vermeer P.A. (1998),
PLAXIS Finite Element Code for Soil and
Rock Analysis, Version 7, Part: Material
Models Manual. Balkema, Rotterdam
Figure 1: Replacement of 130.0 m of soil by
1.0 m of soil with a higher density.
[2] Bjerrum L. (1967), Engineering geology of
Norwegian normally consolidated marine
Question:
clays as related to settlements of buildings.
Why are the stresses of non-porous materials
Gotechnique 17 (2), pp. 81-118.
not shown and how can I visualise them anyway?
14
PLAXIS
PLAXIS
Answer:
Non-porous material is general used for
structural purposes like concrete, and stresses
can therefore become quite large. As a result,
stresses in the soil cannot be viewed in detail.
By excluding the stresses in structural
elements, the soil stresses remain clearly visible.
The stresses in non-porous material are
calculated though and can be viewed in tables
and cross-sections.
However, there is a trick to view the stresses
in non-porous material:
Calculate the problem and if all the calculations
are finished and you want to have a look at the
stresses in output then save the calculation
data by pressing the save icon. Then go directly
to Input and open the concrete material data
set. Change <non-porous> to <drained> and
exit the material data set. Save the Input data
and go directly to Output by pressing the
Output icon. In Output the stresses in the nonporous material set are now visible. Output is
tricked in believing that the non-porous
material set is drained and shows the stresses.
15
PLAXIS
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